1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, and more particularly, it relates to an image forming apparatus in which residual toner remaining on an image bearing member can be collected by a developing device.
2. Related Background Art
Recent copying machines and printers have been digitalized as a full-color image and systematization the apparatus have been required.
For example, laser beam printers in which a photosensitive drum is scanned by a laser beam and a latent image is formed on the photosensitive drum by ON/OFF-control of the laser beam to obtain a desired image have widely been proposed. Such printers are mainly used for effecting two-value recording of characters, figures and the like. Since the recording of the characters, figures and the like does not require intermediate gradation, the printer can be simplified.
There are printers in which the intermediate gradation can be obtained by the two-value recording system. In such printers, it is well-known to utilize a dither method or a density pattern method. As is well-known, in the printers utilizing the dither method or the density pattern method, high resolving power cannot be obtained. However, recently, there has been proposed a method in which intermediate gradation can be obtained for each pixel without worsening high recording density. In such a method, the intermediate gradation is obtained by effecting pulse-width-modulation (PWM) of a laser beam by using an image signal. According to this method, an image with high resolving power and high gradation can be formed.
Now, an example of an image forming apparatus utilizing the above-mentioned method will be explained with reference to FIG. 4.
In FIG. 4, when a copy start signal is inputted, a photosensitive drum 201 is charged to a predetermined potential by means of a charger 203. On the other hand, an original 200G rested on an original support 210 is scanned by illuminating light emitted from a unit 209 comprising an original illumination lamp, a short-focus lens array and a CCD sensor, so that light (from illumination scan light) reflected by the original is focused by the short-focus lens array and is incident on the CCD sensor.
The CCD sensor includes a light receiving portion, a transmitting portion and an output portion. In the light receiving portion, the light signal is converted into a charge signal, and, in the transmitting portion, the charge signals are successively transmitted to the output portion in synchronism with clock pulses. Then, in the output portion, each charge signal is converted into a voltage signal which is in turn amplified and impedance-reduced and then is outputted. An analogue signal so obtained is subjected to conventional image treatment to change a digital signal which is in turn sent to a printer portion.
As shown in FIG. 5, in the printer portion, light emitted from a solid laser element 102 ON/OFF-emission-controlled in response to the image signal is scanned by a polygon mirror 104 rotating at a high speed, thereby forming an electrostatic latent image corresponding to an image of the original on the photosensitive drum 201.
Next, a laser scan portion 100 for scanning a laser beam will be described with reference in FIG. 5.
When the laser beams are scanned by the laser scan portion 100, first of all, the solid laser element 102 is switched (between bright and dark) at a predetermined timing by a light-emitting signal generator 101 in response to the inputted image signal. Laser beams emitted from the solid laser element 102 are converted into substantially parallel light fluxes by a collimator lens system 103. The light fluxes are scanned in a direction shown by the arrow C.sub.0 by the polygon mirror 104 rotating in a direction shown by the arrow b and are focused onto a scanned surface 106 (surface to be scanned) as a spot by means of a group of f.theta. lenses 105a, 105b, 105c.
Exposure distribution corresponding to one scan image is formed on the scanned surface 106 of the photosensitive drum 201. Whenever the scan is effected, by scrolling the scanned surface 106 by a predetermined amount in a direction perpendicular to the scan direction, entire exposure distribution corresponding to the image signal can be formed on the scanned surface 106.
Then, the electrostatic latent image is developed by a developing device 204 containing a two-component developer (including toner particles and carrier particles), thereby forming a toner image on the photosensitive drum 201.
Now, a developing process will be described. Generally, the developing methods are divided into foul methods. In the first method, non-magnetic toner is coated on a developing sleeve by a developing blade to form a toner layer and development is performed without contact between the toner layer and the photosensitive drum (one-component non-contact development). In the second method, magnetic toner is coated on a developing sleeve by a magnetic force to form a toner layer and development is performed without contact between the toner layer and the photosensitive drum (one-component non-contact development). In the third method, developer is constituted by mixture of toner particles and magnetic carrier particles and the developer is conveyed by a magnetic force and development is performed while contacting the toner layer with the photosensitive drum (two-component contact development). In the fourth method, developer is constituted by mixture of toner particles and magnetic carrier particles and the developer is conveyed by a magnetic force and development is performed without contact between the toner layer and the photosensitive drum (two-component non-contact development). Incidentally, the two-component contact development is widely used in the view point of high quality image and great stability.
FIG. 6 is a schematic view showing a developing device 204 of two-component magnet brush type used in the above-mentioned conventional example.
The developing device 204 includes a development container 216, and a developing sleeve 211 disposed within an opening portion of the development container in an opposed relation to the photosensitive drum 201. A fixed magnet roller 212 is disposed within the developing sleeve 211. Further, there is provided a regulating blade 215 for forming a thin toner layer on the developing sleeve 211.
The development container 216 is divided, by a partition 217, into a developing chamber R201 and an agitating chamber R202 including agitating screws 213, 214, respectively. A toner reservoir or hopper R203 is disposed above the agitating chamber R202.
The developing sleeve 211 is arranged in such a manner that a part of the sleeve nearest to the photosensitive drum 201 is spaced apart from the drum by about 500 .mu.m. As shown in FIG. 6, the developing sleeve is rotated in a normal direction together with the photosensitive drum 201 so that the development is effected while contacting the toner with the photosensitive drum 201. A peripheral speed ratio of the developing sleeve 211 relative to the photosensitive drum 201 is normally selected to 1.5 to 2.0 times.
The toner image formed on the photosensitive drum 201 is electrostatically transferred onto a transfer material by a transfer charger 207. Thereafter, the transfer material is electrostatically separated from the photosensitive drum by a separation charger 208, and the separated transfer material is sent to a fixing device 206, where the toner image is thermally fixed to the transfer material. Thereafter, the imaged transfer material is discharged from the image forming apparatus.
After the toner image was transferred, residual toner remaining on the surface of the photosensitive drum 201 is removed by a cleaner 205, thereby preparing for next image formation.
The above-mentioned arrangement is only an example. Thus, the charger 203 may be a charge roller in lieu of a corona charger and the transfer charger 207 may be a transfer roller. However, fundamentally, the image formation is performed through charge, exposure, development, transferring, fixing and cleaning processes.
Recently, compactness of such image forming apparatuses has been promoted and progressed. However, there is the limitation merely by making the charge means, exposure means, developing means, transfer means, fixing means and/or cleaning means. Further, although the residual toner (waste toner) is collected into the cleaner 205, from the viewpoint of protection of environment, the waste toner should be reduced to be as little as possible.
To this end, there has been proposed a cleaner-less (no cleaner) device in which such a cleaner 205 is omitted and the development and the cleaning are simultaneously effected by the developing device 204 (simultaneous development/cleaning). The simultaneous development/cleaning means a technique in which residual toner remaining on the photosensitive drum (after transferring) is removed and collected by fog removing bias V.sub.back during the next developing process.
According to this technique, since the collected residual toner is used in the next and further developing processes, the waste toner can be eliminated. Further, since any waste toner container is not required, space can be saved, thereby permitting remarkable compactness of the apparatus.
However, in the conventional copying machines such as the above-mentioned example, when the cleaner 205 was omitted and the simultaneous development/cleaning was effected, it was found that positive ghost of the previous image is generated at a non-image portion of the photosensitive drum. The positive ghost is a phenomenon caused when a part of the residual toner (used to form the previous image) which is not completely removed is transferred onto a white background of the photosensitive drum.